I. Field of the Invention
The present invention relates to communications circuits. More particularly, the present invention relates to novel and improved techniques for adjusting the gain of variable gain amplifiers.
II. Description of the Related Art
Variable gain amplifiers (VGAs) are commonly used in communications receivers and transmitters to provide adjustable gain, and thus variable signal amplitude, depending on, for example, operating conditions, system requirements, or other factors. In particular, some transmitters are required to provide a range of adjustment in their output power. One application that requires this power adjustment is a Code Division Multiple Access (CDMA) communications system.
In the CDMA system, the signal from each user is spectrally spread over the entire (e.g., 1.2288 MHz) system bandwidth. Thus, the transmitted signal from each transmitting user acts as interference to those of other users in the system. To increase system capacity, the output power of each transmitting remote station is adjusted such that a required level of performance (e.g., a particular bit error rate) is maintained while minimizing interference to other users.
The transmitted signal from the remote station is affected by various transmission phenomena, including path loss and fading. These phenomena, in combination with the need to control the transmit power, can impose a difficult specification on the required transmit power control range. In fact, for the CDMA system, each remote station transmitter is typically designed with the capability to adjust its output power over a range of greater than 85 dB.
The power adjustment for CDMA remote stations is typically provided by one or more variable gain amplifiers in the transmit signal path. The power adjustment is normally performed in increments that are typically specified. For example, for CDMA remote stations that conform to the IS-95-A standard, the output power is adjusted in 0.5 dB increments.
For ease of compliance with the power adjustment specification and for other system considerations, it is advantageous to be able to linearly adjust the gain (in dB) of at least one variable gain amplifier in the transmit signal path. Linear in dB gain versus control voltage corresponds to an exponential gain transfer function. For some VGA designs, the exponential gain is achieved with a differential amplifier stage that provides an output current that varies exponentially in response to a differential input control voltage. The transfer function for the differential amplifier is approximately linear in dB, but compresses at large control voltages.
VGAs having transfer functions that are not linear in dB can cause degraded performance. For example, a xe2x80x9cdistortedxe2x80x9d (i.e., not linear) transfer function can make it more difficult or challenging to accurately set the transmit output power to a particular level. In CDMA systems, this inaccuracy can result in degraded performance for a particular remote station (if the output power is set too low) or lower system capacity (if the output power is set too high). The distorted transfer function can also affect other circuits (e.g., the AGC loop) that depend on the gain slope of the VGA. For example, a non-linear gain versus control voltage can lead to large gain slopes, which allow noise to more easily transfer onto the transmitted signal. The large gain slopes can also disturb the stability of the AGC loop.
Several advantages may be obtained by the use of VGAs that are linear in dB. For example, linear in dB VGAs may reduce the requirement for the RF calibration. Voltages from linear in dB VGAs can also be provided to an AGC loop to generate a log (amplitude) detector, which is useful in some applications.
Accordingly, techniques that allow for linear (in dB) adjustment of a variable gain element are highly desirable.
The present invention provides techniques to linearly (in dB) adjust the gains of variable gain elements (i.e., variable gain amplifiers or VGAs) in a receiver or transmitter. An input control signal is provided to a conditioning circuit that conditions the control signal to achieve various signal characteristics. For example, the input control signal can be limited to within a particular range of values, temperature compensated, scaled (or normalized) to the supply voltages, shifted with an offset, or manipulated in other fashions. The conditioned signal is then provided to an input stage of a linearizer that generates a set of exponentially related signals. This can be achieved using, for example, a differential amplifier in which the conditioned control signal is applied to the inputs of the differential amplifier and the collector currents from the differential amplifier comprise the exponentially related signals. The differential amplifier can be implemented, for example, with a BJT differential air. An output stage within the linearizer receives the exponentially related signals and, in response, generates a gain control signal. By approximately matching the output stage to a gain stage of the variable gain element and by using the gain control signal generated by output stage, the gain transfer function of the VGA can approximate that of the exponentially related signals.
An embodiment of the invention provides a linearizer circuit for providing a control signal for a variable gain element. The linear includes a first circuit coupled to a second circuit. The first circuit is configured to receive an input signal and generate exponentially related signals. The second circuit is configured to receive the exponentially related signals and generate the control signal in response to the received exponentially related signals. The second circuit is approximately matched to a gain circuit within the variable gain element. In a specific implementation, the first circuit or the second circuit, or both, can each be implemented with a differential amplifier. For improved performance, the second circuit can be matched (as closely as possible) to the gain stage of the variable gain element. The input signal can also be conditioned to achieve various signal characteristics such as being limited to within a particular range of values, temperature compensated, continuously adjustable, adjustable in discrete steps, offset vertically in discrete steps, and others.
Another embodiment of the invention provides a gain control circuit for providing a gain control signal for a variable gain element. The gain control circuit includes a conditioning circuit coupled to a linearizer circuit. The conditioning circuit is configured to receive and condition an input control signal to generate a conditioned control signal. The linearizer circuit is configured to receive and process the conditioned control signal to generate the gain control signal. The gain control signal, over a particular range of values, provides an approximately linear, in dB, gain transfer function in the variable gain element. The conditioning circuit can include zero or more of the following circuits: a clipping circuit, a temperature compensation circuit, a summing circuit, and other circuits. The clipping circuit is configured to receive the input signal and provide a clipped signal that is limited to a range of values defined by an upper limit and a lower limit. The temperature compensation circuit is configured to receive an input signal and provide a temperature compensated signal such that the gain transfer function in the variable gain element is approximately invariant to temperature variations. The summing circuit is configured to receive an input signal and a trim signal and to combine the signals to generate a combined signal. These circuits can be arranged in various orders.
Yet another embodiment of the invention provides a circuit for processing an analog signal. The processing circuit includes at least one variable gain element coupled in series and operative to receive an analog signal. Each variable gain element provides a particular gain for the analog signal based on a respective gain control signal. The processing circuit further includes at least one gain control circuit coupled to the variable gain element(s). Typically, one gain control circuit is provided for each variable gain element. One or more of the gain control circuit(s) can be designed to include a linearizer circuit. The linearizer circuit is configured to receive and process an input control signal to generate the gain control signal. The gain control signal, over a particular range of values, provides an approximately linear (in dB) gain transfer function in the variable gain element operative to receive that gain control signal. The gain control circuit can also be designed to include a conditioning circuit coupled to the linearizer circuit. The conditioning circuit can include zero or more of the following circuits: a clipping circuit, a temperature compensation circuit, a summing circuit, and other circuits. The processing circuit can be included, for example, within a receiver or a transmitter that can be used, for example, in a cellular telephone.